Single-pinhole and multi-pinhole SPECT imaging are used increasingly in clinical organ specific studies and preclinical studies using small animals. One advantage of pinhole collimation with magnification is sub-millimeter resolution which is not achievable for parallel-beam SPECT. Multi-pinhole SPECT with overlapping counts is also used to improve sensitivity over single-pinhole SPECT.
In order to achieve high resolution, a multi-pinhole SPECT system requires accurate calibration of its geometric parameters. Among the geometric parameters that may need calibrating are the focal length, radius-of-rotation, pinhole locations, pinhole plate transaxial and axial offset (or mechanical offset), detector center-of-rotation offset (or electrical shift), and twisting and tilting of the plate, and so on.
In practice, assumptions can often be made to reduce the number of calibration parameters, based on the knowledge of a specific system. For example, a conventional calibration approach includes acquiring the projection data of point-source calibration markers, finding the locations of the 2D dots on the projection data, and estimating the geometric parameters by fitting the forward-projected dot-locations to the measured dot-locations, or by some analytic methods.
These conventional methods generally require fairly good knowledge of the 3D coordinates of the point-source markers in order to identify most of the dots on the projection data in terms of what point-source through what pinhole, and to pair up measured dots with dots by forward projection. This sorting procedure is called “dots assignment”. If there are large errors in the 3D locations of point-sources, the dots assignment may fail for most dots, and the calibration may fail as well.
A CT scan is typically required to determine the 3D locations of the calibration markers. This typically increases the cost and time for medical imaging procedures.
Desirable in the art is an improved method of calibrating a multi-pinhole SPECT system.